The present invention relates to a method and apparatus for continuous heat sterilization of liquid. Particularly, the present invention relates to a method for continuous heat sterilization of liquid including a preheating step, a final heating step, a retaining step and a cooling step, and to an apparatus for performing this method.
Conventionally, as a sterilizing method for food, there has been widely adopted a heat sterilization method for heating food at high temperatures. Food is classified into liquid-like food or solid-like food, depending on whether convection is predominant or conduction is predominant in a mechanism of heat transfer when the food is heated, and is also classified into paste-like food as food showing properties between properties of liquid-like food and solid-like food.
As methods of heat sterilization of liquid-like or paste-like food, there is a pre-filling sterilizing method in which food is heated and sterilized before being filled into a container, filled and hermetically sealed in the container while remaining at high temperatures, cooled in a sterilized manner and filled in a sterilized atmosphere into the container in a sterilized state. There is also a post-filling sterilizing method in which food is filled into a container, hermetically sealed, and thereafter heated along with the container to thereby sterilize the food. Further, the former pre-filling sterilizing method is classified into a case in which food is continuously heated, and a case in which food is batch heated by a double can (refer to xe2x80x9cFood Engineering Basic Course, volume 10, Food Reaction Engineeringxe2x80x9d, Kiyoshi Kubota et al., page 111, Korin Kabushiki Kaisha, Sep. 29, 1990).
Generally, when liquid as in liquid-like food or paste-like food is heated and sterilized, there is carried out a continuous heat sterilization method of an HTST method, a UHT method or the like in which mass processing is feasible. An explanation will be given of an example of a conventional continuous heat sterilization method of liquid with reference to FIG. 4. FIG. 4 is a schematic view for explaining an example of a conventional continuous heat sterilization apparatus for liquid.
In FIG. 4, a conventional apparatus 1c for continuous heat sterilization of liquid includes a feed pump 3 connected to a storage tank 2 for liquid. In FIG. 4, a rotary type constant amount pump is used as the feed pump 3. Although the feed pump of FIG. 4 is formed with a pipe for returning liquid from an outlet pipe to an inlet pipe, and the pipes are installed with a flow regulating valve, illustration thereof is omitted in FIG. 4. By returning liquid from an outlet side to an inlet side of the feed pump by the pipes, and regulating an amount of return by the flow regulating valve, pressure on an outlet side of the feed pump or a flow rate of pressure-fed liquid is regulated. On the outlet side of the feed pump 3, there is formed a flow pass of liquid connected with a preheating section 10, a low pressure homogenizer 20, a final heating section 35, a holding pipe 4, a first cooling section 40, a second cooling section 50 and a back pressure regulating valve 5 in this order, and a sterilized liquid outlet 6. Further, all of the preheating section 10, the final heating section 35, the first cooling section 40 and the second cooling section 50 for carrying out heat exchange are constituted by plate type heat exchangers.
Liquid stored in the storage tank 2 is pressure-fed to the preheating section 10 by the feed pump 3. In the preheating section 10 steam is introduced from a steam source 11 and subjected to heat exchange with liquid, and liquid is preheated to reach a predetermined preheating temperature. A temperature sensor 14 is installed at an outlet of the preheating section 10, an opening amount of a steam regulating valve 15 is regulated in accordance with temperature of liquid, and a temperature of liquid is controlled to a predetermined preheating temperature. The preheating temperature is generally about 40xc2x0 to 90xc2x0 C. Vapor-condensed water resulting from heat exchange is discharged via a trap pipe 12 or a discharge pipe 13. Liquid reaching the predetermined preheating temperature is pressure-fed to the low pressure homogenizer 20. A preheating step is carried out by the preheating section 10.
As a principle, the low pressure homogenizer 20 is provided when liquid includes fat, and is an apparatus for homogenizing the liquid by destructing fat balls in the liquid. Generally, a homogenizer is installed with a high pressure pump and a homogenizing valve (not illustrated), in either of a case of a low pressure homogenizer and a high pressure homogenizer, mentioned later. A high pressure pump is an apparatus in which a pressurizing chamber of a predetermined volume, having an inlet and an outlet for liquid, is provided with a plunger reciprocating in directions for increasing and decreasing the volume of the pressurizing chamber, and check valves are installed at the inlet and the outlet of the pressurizing chamber. Volume of the pressurizing chamber is increased and decreased by reciprocation of the plunger, and the check valves are naturally opened and closed in accordance with an increase and decrease of the volume to thereby suck and deliver liquid.
A homogenizing valve is a kind of a throttle valve for regulating a clearance of a flow pass for liquid delivered from a high pressure pump. That is, the homogenizing valve pressurizes liquid by throttling the clearance of the flow pass for liquid. Further, the homogenizing valve makes liquid pressurized in this way pass in a narrow flow pass and releases it to normal pressure, and destructs fat balls in the liquid during this occasion. Generally, pressure caused by narrowing a flow path by a homogenizing valve is referred to as homogenizing pressure. As a principle, homogenizing pressure of the low pressure homogenizer 20 is regulated within a range of from about 2 to 40 MPa.
Further, the low pressure homogenizer 20 is provided with a hydraulic control unit 21. The hydraulic control unit 21 is provided with a hydraulic pump and a hydraulic valve (both of which are not illustrated). Pressure is applied to oil by the hydraulic pump, the pressure is regulated by the hydraulic valve and thereafter, a homogenizing valve is operated by pressure of the oil to thereby regulate homogenizing pressure.
Generally, it is preferable to carry out a homogenizing operation with liquid at high temperature, and the low pressure homogenizer 20 is frequently provided downstream of the preheating section 10, however, there is a case in which the homogenizer 20 is installed downstream of the final heating section 35. In this case, liquid which comes out from the final heating section 35 is cooled, and thereafter made to pass through the low pressure homogenizer 20. Liquid passing through the low pressure homogenizer 20 is liquid after sterilization, and accordingly, the low pressure homogenizer 20 is to be sterilized. The low pressure homogenizer 20 specified to be sterilized, is provided with a structure in which, for example, steam or sterilized water is always supplied to seal portions of the plunger to thereby prevent contamination of liquid, and generally an apparatus cost and running cost are higher than those of an apparatus not specified to be sterilized. Further, the low pressure homogenizer 20 is installed with a bypass pipe 22 which is used only during cleaning for ensuring a flow rate of cleaning solution.
Liquid which comes out from such low pressure homogenizer 20 is fed to the final heating section 35. The final heating section 35 is a heat exchanger for elevating a temperature of liquid to a predetermined sterilizing temperature. The final heating section 35 is provided with a hot water producing device 36 which circulates hot water via a hot water circulating pass 37 to thereby heat liquid to reach the predetermined sterilizing temperature. A temperature sensor 33 is installed at an outlet of the final heating section 35 by which temperature of liquid is detected. Generally, a sterilizing temperature frequently falls within a range of from 90xc2x0 to 160xc2x0 C. A final heating step is carried out by the final heating section 35, mentioned above.
Liquid having reached the sterilizing temperature is flowed through the holding pipe 4. The holding pipe 4 is provided with a predetermined length and holds liquid for a predetermined period of time by which microorganisms in the liquid are exterminated.
This holding step is carried out by the holding pipe 4. Incidentally, temperature of liquid may be lowered by holding heat while flowing the liquid through the holding pipe 4, and is not necessarily maintained strictly at the sterilizing temperature; however, in the following explanation, an intermediary step, after the liquid has reached the sterilizing temperature and until the cooling step is carried out is regarded as the holding step.
Liquid with resulting from sterilization is fed to the first cooling section 40 and the second cooling section 50. In the first cooling section 40, water is fed from a water source 41 to a water pipe 42 to thereby cool liquid. In the second cooling section 50, cold water is fed from a cold water source 51 to a cold water pipe 52 to further cool liquid. Cooling steps are carried out by the first cooling section 40 and the second cooling section 50, mentioned above. Liquid resulting from the cooling steps is fed from the sterilized liquid outlet 6 to a succeeding step, for example, a sterilized filling step or the like.
Further, normally, in the final heating section 35, temperature of hot water in the hot water circulating pass 37 is regulated based on a detection value of the temperature sensor 33, and is automatically controlled to a predetermined sterilizing temperature. Furthermore, at the outlet of the final heating section 35, when temperature of liquid becomes lower than an allowable temperature range, an operation of automatically returning sterilized liquid back to the storage tank 2 is carried out; however, in FIG. 4, illustration of devices for this operation is omitted.
Further, generally, in the continuous heat sterilization apparatus shown by FIG. 4, for energy conservation, there is a case in which heat exchange is carried out between high temperature liquid after sterilization and low temperature liquid before sterilization. Further, as another example of the final heating section 35, there is a type in which steam is directly blown into liquid to thereby heat it, and in this case, a vacuum chamber is used in the first cooling section 40 and there is carried out an operation of cooling liquid by estimating blown steam. However, in the above-described heat sterilization method (hereinafter, described as Prior Art 1), liquid is frequently denatured by being heated and particularly when liquid to be sterilized is food, drugs, or raw materials of these, it is recognized that flavor is changed to some degree during a heat sterilization process.
In order to overcome such difficulties of heat sterilization, in recent years, there has been developed a high pressure sterilizing method. The high pressure sterilizing method is a method of exterminating microorganisms by holding liquid under high pressure, and there are known various technologies for performing such a sterilizing method, for example, technologies disclosed in JP-A-4-174669, JP-B-6-57236, JP-A-6-225707, JP-A-6-327445, JP-A-6-327446, JP-A-8140593, JP-A-8-196249, JP-A-57-22679, JP-A-4-91770, JP-B-7-28706 and JP-A-5-227925 (hereinafter, described as Prior Art 2).
Further, for the high pressure sterilizing method, particularly, as a method of using an ultra high pressure homogenizer, there is known a technology disclosed in JP-A-6-205655 (hereinafter, described as Prior Art 3).
Further, the inventors have invented a sterilizing method in which liquid is pressurized, thereafter press-fed to two divided flow passes, and injected from small holes opposed to each other to thereby collide with each other, by which sterilization is carried out, and the inventors have already filed an application patent (JP-A-7-298861, hereinafter, described as Prior Art 4).
However, in the above described Prior Art 1, as the final heating section 35, there is frequently used a heat exchanger of a type in which heat of a heat medium and liquid is transferred and received via a heat transmitting wall, for example, a plate type heat exchanger. Furthermore, there is frequently used a steam nozzle of a type in which steam is blown into liquid to thereby heat it, and accordingly, the following problems are posed.
i) In the final heating section 35, it is inevitable that scorch (scale) is adhered to a heat transmitting wall or the steam nozzle, and efficiency of heating is deteriorated with elapse of time.
ii) A period of operational time is restricted by scorching of the final heating section 35. That is, when a large amount of scorch is adhered to the final heating section 35, sterilizing processing needs to stop and accordingly, continuous operation for a long period of time is not possible.
iii) Particularly, in the case of a plate type heat exchanger, temperature is gradually elevated until liquid reaches a sterilizing temperature, and denaturation of liquid is progressed by excessive heating while a temperature of the liquid is elevating.
According to the high pressure sterilizing method of Prior Art 2 mentioned above, there poses a problem in which generally, pressurizing to a high pressure of at least 100 MPa is needed, and accordingly, an apparatus having high strength is needed. Furthermore, a time period of at least 1 minute is required, and therefore, as a whole, investment cost is enormous and an amount of processing is small, which is unsuitable for mass production. In other words, according to the above-described Prior Art 2, when a processing pressure is set low a processing time period is prolonged, and when a processing pressure is set high more strength of the apparatus is needed, and as a result, there is not present any apparatus suitable for mass production while investment cost is low.
According to an apparatus of using an ultra high pressure homogenizer of Prior Art 3, a homogenizing pressure is an ultra high pressure exceeding 100 MPa, and accordingly, a special exclusive homogenizer is needed. Further, an effect of sterilization is poor in comparison with that of heat sterilization, and accordingly, it is difficult to achieve complete extermination of bacteria, and a liquid having low pH of exclusively juice or the like is an object of the apparatus.
Although Prior Art 4 resolves the above-described problem, an apparatus having a slightly complicated structure is needed, and accordingly, a simpler method and apparatus have been long-awaited.
As a result of continuing intensivey research to improve the apparatus of Prior Art 4 so as to make it more simple, the inventors have discovered that this problem can be resolved by a simple apparatus using an on sale homogenizer, and have completed the invention.
It is an object of the invention to provide a method for continuous heat sterilization of liquid in which scorch during a final heating step is dispensed with, and accordingly, a long period of continuous operation is possible. Further, denaturation of liquid caused by heating is insignificant.
It is another object of the invention to provide an apparatus for continuous heat sterilization of liquid in which scorch during a final heating step is dispensed with, and accordingly, a long period of continuous operation is possible. Furthermore, denaturation of liquid caused by heating is insignificant. Still further, investment cost and running cost are low and an amount of processing is large.
According to a first aspect of the invention, there is provided a method for continuous heat sterilization of liquid, includes a continuous preheating step of preheating the liquid, a final heating step of heating the preheated liquid to reach a predetermined sterilizing temperature, a holding step of holding the liquid which has reached the predetermined sterilizing temperature for a predetermined period of time, and a cooling step of cooling the held liquid, wherein the final heating step comprises the following steps (a) and (b):
(a) a step of continuously pressurizing the liquid by a high pressure pump; and
(b) a step of releasing the liquid continuously to a normal pressure within a time period of less than 10 seconds after the pressurizing step to thereby make the liquid reach the predetermined sterilizing temperature.
Preferable modes of the first aspect of the invention are that the steps (a) and (b) are carried out by a homogenizer (hereinafter, described as a first mode), and that during step (a) the liquid is pressurized within a pressure range of from 50 MPa to 100 MPa (hereinafter, referred to as a second mode), and that during step (b) the liquid is made to reach a temperature range of from 90xc2x0 C. to 160xc2x0 C. (hereinafter, described as a third mode).
According to a second aspect of the invention, there is provided an apparatus for continuous heat sterilization of liquid, which apparatus comprises a storage tank for storing the liquid, a feed pump for press-feeding the liquid stored in the storage tank, a preheating section for preheating the liquid press-fed by the feed pump, a pressurization release apparatus including a high pressure pump and a throttle valve for continuously pressurizing the liquid preheated by the preheating section and releasing the liquid preheated by the preheating section to a normal pressure continuously within a time period of less than 10 seconds to thereby reach a predetermined sterilizing temperature, a holding pipe for holding the liquid which has reached the predetermined sterilizing temperature, and a cooling section for cooling the liquid held by the holding pipe.
Preferable modes of the second aspect of the invention are that the continuous heat sterilization apparatus further comprises a temperature sensor provided at the holding pipe for detecting a temperature of the liquid coming out from the pressurization release apparatus, a controlling device for comparing a detection value detected by the temperature sensor with a previously inputted predetermined sterilizing temperature and calculating an operational amount for making the detection value and the predetermined sterilizing temperature approach each other, and a pressure regulating device for increasing or decreasing an amount of pressurization of the pressurization release apparatus in accordance with the operational amount calculated by the controlling device, whereby a temperature of the liquid coming out from the pressurization release apparatus is automatically controlled to the predetermined sterilizing temperature by the pressure regulating device (hereinafter, described as a fourth mode).
The continuous heat sterilization apparatus alternatively comprises a temperature sensor provided at the holding pipe for detecting a temperature of the liquid coming out from the pressurization release apparatus, a controlling device for comparing a detection value detected by the temperature sensor with a previously inputted predetermined sterilizing temperature and calculating an operational amount for making the detection value and the predetermined sterilizing temperature to approach each other, and a device for regulating a temperature of an inlet of the pressurization release apparatus for increasing or decreasing a temperature of the liquid on a side of the inlet of the pressurization release apparatus in accordance with the operational amount calculated by the controlling device, whereby a temperature of the liquid coming out from the pressurization release apparatus is automatically controlled to the predetermined sterilizing temperature by regulating a temperature of the liquid on the side of the inlet of the pressurization release apparatus (hereinafter, described as a fifth mode). Also, the pressurization release apparatus can be a homogenizer (hereinafter, described as a sixth mode).